Unraveling CASTOR1's tumor suppressive role: Genetic ablation of CASTOR1 and KRAS-driven mouse model of non-small cell lung cancer.

Abstract

8528 Background: Cytosolic Arginine Sensor for mTORC1 Subunit 1 (CASTOR1) has emerged as a potent suppressor of mTORC1 activity, playing a pivotal role in various cancer contexts. Previously implicated as a tumor suppressor in multiple cancers, CASTOR1 is tightly regulated in cancer, including inhibition by viral miRNAs in KSHV-induced cancer and degradation via AKT-phosphorylation and RNF167-mediated ubiquitination. Notably, CASTOR1's role extends beyond normal cellular contexts, as evidenced by its tumor-suppressive function in breast cancer xenograft models and its prognostic value in lung adenocarcinoma (LUAD) and squamous cell carcinoma (LUSC). This study aims to elucidate CASTOR1’s role in a transgenic Kras-driven LUAD mouse model, shedding light on its tumor-suppressive potential. Methods: Utilizing a novel CASTOR1 knockout (KO) mouse model crossed with the Kras (LSL-KrasG12D/+) NSCLC model, we established a new mouse model with simultaneous Kras activation and homozygous CASTOR1 ablation. Tumor incidence and growth were monitored in Kras mice harboring wild-type (WT) and KO CASTOR1. Further investigations examined the proliferative dynamics and activation status of various oncogenic pathways, and assessed mTORC1 inhibitor sensitivities in human LUAD cell lines with altered CASTOR1 expression levels. Results: CASTOR1 deficiency markedly heightened lung tumor incidence and size, concomitant with an elevated proliferative index. Intriguingly, a novel antibody identified robust pCASTOR1 levels in CASTOR1 WT mouse LUAD tumors, positively correlated with tumor stage and inversely related to total CASTOR1 abundance, hinting at an active degradation mechanism promoting tumor progression. Elevated mTORC1 signaling, evidenced by increased levels of p4EBP and pS6, underscored the pathway's activation consequent to CASTOR1 loss, likely driving tumor development. Furthermore, heightened pERK levels in CASTOR1 KO tumors unveiled a novel feedback loop intertwining CASTOR1/mTORC1 and the Kras/ERK pathway. Notably, treatment with mTORC1 inhibitors sensitized organoids of Kras tumors with mTORC1 hyperactivation to Kras G12D inhibitor. Conclusions: Our findings elucidate CASTOR1's tumor-suppressive role in Kras-dependent mouse LUAD tumors, suggesting the potential efficacy of combined Kras and mTORC1 inhibition in LUAD subsets characterized by hyperactivated Kras and diminished CASTOR1 levels or mTORC1 hyperactivity.